Various types of metal materials such as sheets, coils, molded articles are subjected to surface treatment for providing various types of performance. Examples of the performance include, preventing corrosion, making coatings less likely to be peeled, making coatings more likely to be wet with water, and making coatings more likely to repel water. Above all, aluminum and alloys thereof (hereinafter, referred to as aluminum materials) are generally light and excellent in workability, and thus often used for home electrical appliances and automobiles. Corrosion resistance, hydrophilicity, moisture resistance, and sustainability thereof are required, in particular, for fins and heat exchangers that are used by taking advantage of the characteristically high thermal conductivity of the aluminum materials.
Air-conditioners (including outdoor units) that have functions such as cooling, heating, and dehumidification are provided with fins for heat exchangers, which are formed from the aluminum materials. The fins for heat exchanges are designed so that radiating parts and cooling parts occupy as large surface areas as possible, and intended to improve the radiation effect and the cooling effect. As a result, the fins which serve as radiating parts and cooling parts are often made to have an extremely narrow space therebetween.
The aluminum materials, etc. constituting the fins are often provided with hydrophilicity and corrosion resistance by surface treatment. Methods for the surface treatment include two methods of: a so-called post-coating method of molding or assembling a fin material into an intended shape, and then applying a surface treatment agent by means such as dipping, spraying, or showering; and a so-called pre-coating method of forming, on a plate material, a surface treatment film in advance by means such as a roll coater, and then carrying out molding or the like into a fin material. In the case of the pre-coating method, such a surface treatment coating film is required that will not abrade tools such molds in processing.
In the air-conditioners in operation for cooling, moisture in the air builds up condensation on the surfaces of the fins which serve as cooling parts. The dew condensation water is more likely to become water droplets as the surfaces of the fins are more highly hydrophobic, and the growth of the water droplets causes clogging (bridge) between the fins. When the bridge is caused, problems may be created such as an increase in ventilation resistance, a decrease in heat-exchange efficiency, and dispersal of the clogging water droplets.
In addition, through the intermittent use of the air conditioners, retention of dew condensation water and drying are repeated, and the fin surfaces may be corroded to shorten the product lifetimes. Moreover, in the case of air-conditioners in cars and rooms, corrosion products deposited between fins may fly apart in the cars and the rooms.
Surface treatment coating films provided on the surfaces of the fins may be dissolved or run off by the dew condensation water, and as a result, various types of performance such as hydrophilicity may be lost. In particular, hydrophilic surface treatment coating films that use water-soluble polymers has this tendency remarkably, and the hydrophilicity is likely to have a trade-off relationship with water resistance or durability of hydrophilicity. Thus, the surface treatment coating films are required to have both hydrophilicity and water resistance.
Therefore, in order to solve the problems with dew condensation water, methods of providing corrosion resistance and hydrophilicity are required for the aluminum materials constituting the fins. In particular, methods are required which have excellent hydrophilicity and high corrosion resistance, and retained hydrophilicity and water resistance over a long period of time.
As such surface treatment methods, various methods are proposed such as: a method of spraying a treatment liquid containing silica sol or silicate to forma hydrophilic coating film (Patent Literature 1); a method of forming a chromate-based corrosion-resistant coating film, and then immersing the film in an aqueous solution containing, as its main components, a silicon oxide and a sodium oxide to form a hydrophilic coating film (Patent Literature 2); a method of applying a coating material including a hydrophilic acrylic polymer and an inorganic particulate filler (Patent Literature 3); and a method of applying a coating material composed of a water-soluble or water-dispersible organic resin, an organic corrosion inhibitor, and silica particles (Patent Literature 4).